Extrusion die with adjusting sliding die lips and method of use

ABSTRACT

A thermal actuator system controls the position of a moveable die lip (32, 34) on a flat film extruder die (30) to control the gauge of the extruded plastic film, and includes thermal actuators each of which has an outer thermal bolt (70) and an inner bolt (80) extending through the outer bolt. The inner and outer bolts have outer ends which are connected together, while the outer bolt is connected to the die body and the inner bolt has an operative end connected to a die lip. An air passage (90) is formed between the inner and outer bolts and through which air is applied for cooling the inner bolt with respect to the outer bolt. Thermal expansion of the outer bolt is controlled by an external heater (75) mounted on the outer bolt.

This application claims benefit of provision application 60/010940,filed Jan. 31, 1996.

BACKGROUND OF THE INVENTION

This invention relates to the field of controlling the extrusion gapbetween a pair of die lips for forming sheet material, as an example.

It has been well known to move one or more die lips with respect to theanother in response to the measurements taken by a downstream guage orthickness measuring apparatus, to control the thickness of theextrudate. A widely used arrangement includes the selective heatingand/or cooling of extensible die bolts, one end of which is anchored andthe other end of which causes the flexure of one die lip with respect toanother die lip. Such arrangements are shown in Nissel U.S. Pat. No.3,940,221, Cloeren et al. U.S. Pat. No. 5,020,984 and in Hatori et al.U.S. Pat. No. 5,051,082, as examples.

A particular disadvantage of the apparatus as shown, for example, in theabove referenced patents is the fact that the hinged or flexible dielip, when biased or flexed differentially, such as at a series oflongitudinal positions along the die, causes a distortion in the gap andtherefore causes a non-uniformity with respect to the surface of adeckle rod which may be positioned within the die. Such distortions andlack of uniformities can cause leakage past a deckle rod.

External sliding as adjustable die lips arranged as opposed pairs, havebeen used in combination with die bodies. These die lips provide opposedextrusion land surfaces which are in alignment with an extrusion slot inthe die body, but are separate from the die body itself, so that the dielips may be mechanically adjusted by relative sliding movement alongbeveled lower surfaces of the die body, to open or close an extrusiongap. Typical externally mounted and adjustable die lips, in combinationwith a deckle rod system, are shown in Maejima U.S. Pat. Nos. 4,248,578and 4,659,302. In these patents, an internal seal at the extrusion gapis formed with the sloping inside surfaces of the adjustable die lips,such as rod 6 and die lip 17 of patent U.S. Pat. No. 4,248,579. Anadvantage of this known type of system is that adjustment of theextrusion gap does not cause a change in geometry of the seatingsurfaces of the deckle rod on the sloping inside surfaces of the dielips. To Applicant's knowledge, no prior satisfactory arrangement hasbeen proposed by which a solid sliding external die lip of the kindshown in the Maejima patents has been actuated by thermal actuation.

A further difficulty and limitation inherent to many of the priorthermal bolt actuators resides is the fact that the die slot or gap isclosed by the application of heat which expands the die bolt. At thesame time, the heat is not effectively isolated from the die body andthe heat causes a decrease in the polymer viscosity within the extrusionslot. This decrease in polymer viscosity runs counter to the decrease inslot width, by tending to increase the rate of polymer flow.Accordingly, accurate control of guage thickness can become difficult toachieve.

U.S. Pat. No. 5,051,082 describes a system in FIG. 11 of that patent inwhich, working through a flexible or bendable die hinged die lip, thegap is increased with increasing die bolt temperature and the gap isdecreased with decreasing die bolt temperature, in combination with anairflow arrangement for cooling the die bolt. The die bolt itself isformed with an aperture throughout a major portion of its lengthproviding a passageway for the air to flow for cooling the die bolt. Thecooling air is brought into the die bolt by an air feed tube which iscommon to all of the die bolts. This arrangement puts all of the diebolt cooling systems in series with each other and at a constant coolingbias, with the result that any one die bolt could be heated or cooled ata time when it needs to be heated. Further, heat migration, which isundesirable for control purposes, is intentionally allowed to flow intothe die body so that the viscosity of the molten resin is changed at thesame time that the die lip is undergoing adjustment.

SUMMARY OF THE INVENTION

The invention is directed to apparatus and method for the controlling ofthe relative position of one or more rigid slidable die lips on anextrusion die, using a novel thermal die bolt actuator module. Theinvention is also directed to an improved actuator system. Whileparticularly adapted for use in the positioning of an external slidingdie lip, the system could also be used for the positioning or control ofa flexible integral die lip.

A particular advantage of the actuator system of this invention is thefact that it can generate the force which is required to both bend andslide a portion of the rigid die lip on the die body, to effect a changein die gap. As previously mentioned, this sliding (or selective sliding)of the die lip does not adversely effect the trueness of the sealingsurface with the sealing rod or deckle rod, as employed in an internallydeckled extrusion die. The design of this invention can generate a forcesufficient to bend the rigid die lip along its thicker longitudinalaxis, while the strength of the actuator and the modulus of thecomponents prevent permanent deformation of the actuator rods andcomponents. Each of the die bolts modules is individually heated andindividually cooled, as required, and are isolated from each other, andfrom the die body.

The die bolt actuator module of the present invention has a heatedportion but it is not that portion which is connected to the die lip. Atubular or annular heated bolt surrounds an inner non-heated rigid boltand the non-heated bolt or inner bolt is connected to the die lip, whilethe concentric bolts are joined together at their remote ends.Therefore, the heat which is applied to the outer annular bolt is notdirectly applied to the die lip but rather is isolated by an air gapbetween these inner and outer members. This air gap or chamber isprovided with a cooling air inlet at its lower end and with cooling airoutlets at its upper end, thereby providing a flow path in the spacebetween the bolts for the cooling of the outer or annular heated bolt,when required, and the isolation of the non-heated inner bolt.

The module arrangement is such that the heat which migrates from theouter bolt will be first to the large mass of the screw support bar andsecondly to the even larger mass of the die body itself and, as aresult, will have very little effect on the polymer viscosity, or on theflow rate dependent on viscosity. Thermal variations and disturbancesare held to a minimum.

The components are designed in the assembly or module always to maintainthe parts under an elastic deflection within the limits of the materialused. The length of the bolts are designed such that even with norelative lip deflection, the bolt will remain below its yield deflectionlimit and will not be permanently deformed.

The invention contemplates applying die bolt modules to die lips on bothsides of the die, but offsetting their center lines from the front tothe back of the die thereby allowing 1× spaced control centers with arigid sliding die lip, with the die bolt modules on each side of the dieat 2× center spacing.

The system design is such as to be able to withstand full expansion orcontraction movement of the outer (annular) bolt with no movement of theinner bolt and without sustaining any damage to the actuator module. Itis also necessary to limit the gross or total amount of force applied toone of the sliding die lips to prevent permanent deformation of the lipand, at the same time, prevent the actuator from having a permanentdeformation. The predetermined expansion/contraction of the systemdictates the length of the die bolt system to ensure that the maximumstress which the modules will experience remains within the elasticlimit of the material used.

It is accordingly an important object of the invention to provide anactuator module particularly adapted for the control of the position andbending of a sliding external die lip, in which an outer annular boltsurrounds an inner non-heated bolt thereby defining an airspacetherebetween forming a cooling passageway for air. The outer bolt, whenheated, operates via the inner bolt to increase the extrusion gapbetween the die lips, and when cooled, to decrease the extrusion gap.

Another object of the invention is the provision of a system forsliding, accompanied by bending, of a longitudinally rigid die lipattached to or supported on an exterior surface of a die.

A still further object of the invention is the provision of a forcemodule for positioning a die surface, in which migration of heat to thedie body and/or to the die lips is maintained or held at a minimum.

Other objects and advantages of the invention will be apparent from thefollowing description, the accompanying drawings and the appendedclaims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an end view of a flat film extrusion die having relativelyrigid external die lips and one embodiment of thermal modulator oractuator in accordance with this invention;

FIG. 2 is a sectional view of a preferred embodiment of the thermalactuator die bolt, and showing the outline of the die in broken lines.

FIG. 3 is a side elevation of the actuator support bar of FIG. 2 showinga plurality of the actuators in assembled relation on the bar;

FIG. 4 is an elevational view of the bar assembly of FIG. 3;

FIG. 5 is an enlarged fragmentary sectional view through the die bodyand the die lips showing the extrusion lands or slots in relation to apair of external slidable die lips and also showing the relation of thedeckle rod with respect to the tapered seal surfaces of the die lips;

FIG. 6 is a fragmentary sectional view through a prior art die bodyhaving an integral flexible lip and a thermal actuator die bolt andillustrating how distortions in the die lip adversely affect sealing ata deckle rod;

FIG. 7 is a view similar to FIG. 1 of a further embodiment of anactuator in accordance with this invention, with parts shown partiallyin section;

FIG. 8 is a fragmentary side view of a flat film die with a bank ofactuators for controlling the position of a die lip according to FIG. 7;

FIG. 9 is a sectional view through the actuator of FIG. 7 lookinggenerally along the line 9--9;

FIG. 10 is a fragmentary end view of the actuator of FIG. 7 as mountedon an extrusion die, with the air bar attached;

FIG. 11 is an enlarged fragmentary view of FIG. 10 showing details ofthe attachment of the air bar;

FIG. 12 is a fragmentary side view similar to FIG. 8 and showing theattachment of the air bar to the actuators; and

FIG. 13 is a graph of die lip temperature with variations of thermalactuator temperature.

DESCRIPTION OF FIG. 6

FIG. 6 is representative of a substantial body of prior art. The twopiece die body parts 10 and 12 are joined along a common line 14 anddefines an extruder slot 16 therebetween. A fixed die lip 17 opposes amoveable die lip 18. The lip 18 is moveable through a integral hinge 19.A thermal die bolt 20 is positioned on the body part 12 to bear againstthe lip 18, and the movement or actuation force may be one which closesthe die lips with increasing heat or, as described above in connectionwith patent U.S. Pat. No. 5,051,182 (FIG. 11), opens the die lip withincreasing heat. In either case, a deckle rod 22 must seat against awall of the lip 18 and against a common secondary manifold wall 24.Since the forces applied by the longitudinally spaced series of diebolts 20 is not uniform, the die lip sealing surfaces are distorted byreason of the non-uniform application of a bending force of the die lip18. This lack of uniformity creates a non-planar surface in the wall 24which can cause leakage past the deckle rod 22.

DESCRIPTION OF PREFERRED EMBODIMENTS

Referring first to FIG. 1, an end view of a flat film die 30 isillustrated, in which a pair of external sliding die lips 32 and 34 aresupported. The die lips 32 and 34, as better shown in FIG. 5, defineopposed land surfaces 38 which form an extrusion gap or slot. These dielips adjustably slide on the die body along inclined surfaces 40 whichare mated with corresponding exterior surfaces of the mating die bodyportions 42 and 43. The body portions together, define therebetween aextrusion gap or slot 44 which is wider than the slot 45 defined by theopposed surfaces 38 of the die lips. As shown in FIG. 5, the deckle rod48 rests on the converging die lip surfaces 40, and movement of eitherof the die lips 32 or 34 in a plane parallel to the abutting surfaces ofthe body portions 42 and 43 does not disturb the seated relation of thedeckle rod 48 with respect to the surfaces 40.

The thermal actuators 50 of this invention, as illustrated in FIGS. 1and 2, are spaced longitudinally along one or both of the die bodyportions, such as the portion 34, on an elongated actuator clamping andsupporting bar 52. FIGS. 3 and 4 show the elongated bar 52 supporting aplurality of the actuators 50 thereon.

It is important that the actuator 50 be firmly secured to the actuatorsupport bar 52 and that the actuator support bar 52 be firmly securedand locked to one of the die body portions, such as by bolts (notshown).

The actuator 50, in its preferred embodiment, is described in greaterdetail in connection with the sectional view of FIG. 2. In FIG. 1 it isshown as having exterior threads 55 which are received within internalthreads within the support bar 52. In the embodiment as shown in FIG. 1,the bar 52 is split at 57 to form a gap, and a clamp, by which theactuator 50 may be secured to the bar, by a tie bolt through theaperture 58. Such a bolt would be accessible to the operator, andpermits a retrofit into existing lines in which automatic actuation hasnot been available.

The die lips are mounted to the die body by means of shouldered bolts 60(FIG. 2) which provide for limited sliding movement of the die lipsalong the surface 40 to widen or close the gap or slot 45 definedbetween the opposed surfaces 38. Either or both of the die lips 32, 34may be moved by the actuator 50 of this invention.

In the adjusting movement of any of either of the die lips 32 or 34, inorder to effect guage adjustments along the full longitudinal length ofthe die, it is necessary to exert a force in a plane parallel to theassociated surface 40 at longitudinally spaced positions along suchplane. The die lip 32 or 34 will be caused to be bent or distortedsomewhat about its long axis in making these guage adjustments, asnecessary. And the actuator components are operated under stress withinthe limits of the actuator components.

A preferred form of an actuator according to this invention is shown inthe sectional view of FIG. 2. The actuator support bar 52a is amodification of that shown in FIG. 1 in that, in lieu of the slit 57 andthe clamping function, the actuator bar 52a is provided with a tappedopening for receiving a set screw backlash screw 62 which may be run inand into physical engagement with the actuator body, at the externalthreads, and locked by a lot lock nut 63 to provide an anti backlashattachment.

The improved thermal actuator of this invention, is shown in FIG. 2,includes an outer annular heat extensible bolt 70 forming a body of theactuator. A portion of the outer surface of the annular bolt 70 isthreaded at 72 to be received within the bar 52a for adjustablysupporting the actuator in relation to the position of the associateddie lip.

The outer annular bolt 70 supports, along its length, an electric heaterdiagrammatically illustrated at 75, by which the die bolt 70 may beheated.

The actuator also includes an inner bolt 80 received within the annularbolt 70 and having an inner end 82 extending through the inner end ofthe annular bolt. The inner bolt 80 also has an outer end 84 whichextends outwardly of the outer end of the annular bolt through anopening in the enlarged outer end 85 and secured in a predeterminedadjusted position to the outer end 85 of the annular bolt by a jamb nut86 threaded on the inner bolt. In this manner, the respective outer endsof the inner and outer bolts are mechanically joined together.

The inner and outer bolts define therebetween an air gap or space 90through which cooling air may flow along the inner bolt to isolate andcool the temperature of the inner bolt 80, with respect the outer bolt70.

A cooling air inlet collet 95 is received over the lower end 82 of theinner bolt 80 and sealed by a peripheral seal 96 and maintained inposition by a compression spring 97 threaded over the inner end 82. Thecollet 95 provides an air inlet leading into the interior passageway 90for flow along the length of the inner bolt. The outer end of theannular bolt is provided with one or more air outlet openings 98 throughwhich cooling air may be expelled.

The lower or inner end 82 of the inner die bolt 80 is threaded into theassociated die lip 32 or 34 and an adjusted position may be establishedby a set screw 99 through the die lip body and into the notch 92 on theend 82.

As an example of operating conditions, the cooling passageway 90 and theair flowing therethrough from the inlet collet 95 to the outlet openings98 may provide a temperature of the inner bolt 80 not much in excess ofthe die temperature, typically about 300° F. At the same time, the outerannular bolt 70 may be heated by the heater 75 to a temperature of 900°F., for example. The linear expansion of the outer bolt results in alifting of the respective ends 84, 85 of the bolts and a pulling actionof the inner end 82 with respect to its attachment to the associated dielip. Therefore, the application of heat results in a widening of theextrusion gap 45 and the removal of heat results in the narrowing of theextrusion gap without substantially effecting the temperature of the diebody or the die lips or the viscosity of the material being extrudedtherethrough.

A plurality of the actuators may be positioned along the bar 52 or 52ain spaced relation such as on 2 inch centers. For finer control, acorresponding group of actuators may be applied to the opposite die lipand located on centers which are intermediate those of the first set ofactuators.

As noted above, the individual actuators 50, in operation, are designedso that they cannot be stressed beyond the elastic limit of thecomponents, particularly the elastic limit of the inner or outer bolts,to cause a permanent deformation in such parts. The die lip body 32 (or34) during adjustment, may be caused to be bent or distorted along itslong axis, that is its axis parallel to the plane of force applicationby the actuators, in order to accommodate fine adjustment of the guageof the film extrudant at any given transverse location of the die slot.With the actuators according to this invention, this is accomplished byapplying forces to the die lip, by adjusting the power applied to therespective heaters 75 while maintaining a constant cooling bias by flowof air along the inner bolt 80. The strengths built into the actuator inthe inner and outer die bolts at the actuating temperatures are greaterthan the bending and distortional forces applied by or to any singleactuator so that, under extreme conditions, the actuator componentscannot be stressed beyond their elastic limits. Accordingly, a fewer orgreater number of actuators may be employed, for any die lip, asillustrated in FIGS. 3 and 4, as required.

It will also be apparent that the application of heat to the outer diebolt results in the application of a tensile force to the inner diebolt, and this force is applied in a direction to cause the die lip tobe moved along its plane of movement away from the opposed die lip, towiden the extrusion gap and increase the gauge of the film beingextruded. Similarly, removal of heat energy from the outer die boltcauses a decrease in tensile force or even a compressive force to beapplied to the inner die bolt which is transmitted by the inner die boltto cause the inner die bolt to move in such a direction as to decreasethe film gauge.

Referring to FIGS. 7 and 8, which illustrate a further preferredembodiment of the actuator and system of this invention, a flat filmextruder die is illustrated generally at 100, in end view, havingopposed die bodies 101 and 102 defining therebetween a common extrudermain passage 103 and a common die slot 104. A pair of die lips 110 and112 are mounted respectively on the bodies 101 and 102, on bottommutually sloping and intersecting surfaces, and are moveable in planesparallel to these mounting surface for regulating the width of anextrusion gap 115.

For the purpose of this embodiment, the flat film extrusion die 100 andthe associated die lips 110 and 112 are the same as that which has beendescribed above in connection with FIGS. 1 and 2 for the die 30 and therespective die lips 32 and 34. Thus, the die lips 110 and 112 are in theform of transversely elongated bars which extend the width of the dieand which can be moved for adjustment in a direction parallel to theirrespective sloping or inclined mounting surfaces, as previouslydescribed in connection with the die lip surfaces 40 and the abuttingbody portions 42 and 43 of the embodiment of FIGS. 1 and 2.

An improved thermal actuator according to this invention is illustratedgenerally at 150 in FIGS. 8 and 8. The actuators 150 are spacedlongitudinally along one of the die body portions 101 or 102, such asthe die body portion 101 illustrated in FIG. 8, as mounted on anelongated mounting bar 152. The mounting bar 152 has a portion receivedwithin a slot 153 formed in the inclined outer lower surface of the bodyportion 101 and is firmly locked in place by the mounting bolts 154.

The bar 152 forms the clamping means by which the actuators 150 arefixably mounted to one of the die body portions. The bar 152 isgenerally rectangular in shape and is provided with transverselyextending openings into which the fasteners or bolts 154 (FIG. 8) arethreaded into the die body portion 101, to hold the bar 152 fixably intoposition, as shown. The bar 152 is further provided with a plurality ofintervening tapped and threaded openings 155 for receiving the threadedinner end 160 of an outer annular heat extensible or thermal bolt 170.

As shown in the sectional view of FIG. 9, the inner end 160 of the bolt170 is threaded so as to be received within the threaded bar opening154, and is shouldered at 173 to define a seated position of the boltwith respect to the bar 152. This embodiment differs from that of FIGS.1 and 2 in that the outer bolt 170 is not, itself, adjustable on the diebody but is seated on the bar 152. The die lip adjustment is provided atthe coupling between the inner and outer bolts, as described in furtherdetail below.

The outer annular bolt 170 supports, along a portion of its length, anexternal electric heater 175 by which the die bolt 170 is heated. Theheater 175 is tightly clamped about an outer surface of the outerannular die bolt, and power may be applied to the heater 175independently of power applied to the heaters of the other actuators150.

The actuator 150 also includes an inner die bolt 180 which isproportioned to extend through the outer bolt 170 with a lower or innerend which extends below the bolt 170, through the threaded inner end160. The bolt 180 has a threaded inner or lower end 182 which extendsthrough the opposite end of the bolt 170 below the support bars 150.

The extreme lower threaded end 182 of the inner bolt 180 is threadedinto an aligned opening formed in the die lip 110. Its position isaccurately and adjustably maintained by a lock or jam screw 183 in thedie lip and bears against a flat formed on the end 182. Also a jam nut184 is threaded on the end 182 and bears against the body of the die lip110.

The outer or opposite remote end 185 of the inner bolt 180 extendsoutwardly of and above the outer annular bolt 170. The respective outerends of these bolts are mechanically and adjustably coupled together ata thimble 190. The thimble is formed with an internal recess 192 whichis internally threaded to be received over the threaded outer or upperend 193 of the bolt 170, and is adjustably positioned with respect tothe bolt by the position of a jam nut 194. The thimble 190 is alsointernally threaded at 195 to receive the threaded outer end 185 of thebolt 180 therethrough, which position is maintained by a further jam nut196 on the threaded end 185 and in engagement with the upper surface ofthe thimble 190. Accordingly, the relative position of the inner bolt180 may be adjusted by relative rotation of the thimble 190 with respectto the bolt 180, and locked into position by the jamb nut 196.Similarly, the thimble itself may be positioned with respect to theouter bolt 170 by rotatably adjusting its position and locking the samewith a jamb nut 194. This arrangement creates a differential threadassembly to enable very fine adjustment.

The bar 152, at each threaded actuator receiving opening 155, isprovided with an air inlet opening 200. The lower or inner end 160 ofthe outer annular bolt 170 is formed with a circumferential air inletrecess 204 and with four quadralaterally spaced radial air inletopenings 205 through which air may flow into the interior of the annularbolt. The bar opening 154 is closed by a plug 207 which is threaded intothe bottom end of the support bar 152 and which is flanged to form aclose clearance fit with the lower end of the inner bolt 180, to form anair seal. The plug 207 also acts as an internal jam nut to remove threadbacklash between outer bolt 170 and bar 152.

The major length of the inner bolt 180 between the threaded ends 182,184 is rectangular in cross-section, as shown in the sectional view ofFIG. 9. The corners 210 of the rectangular section form a closeclearance fit with the inside wall 211 of the annular bolt while thespaces therebetween identified at 212 form an air passage which extendsthe entire length of the outer or annular bolt and extends a majorportion of the length of the inner bolt 180 for the passage of coolingair over the inner bolt. The close clearance fits between the corners210 and the cylindrical surface 212 provides mechanical compressivesupport for the inner bolt. Air escape openings 215 are provided in thewall of the thimble 190 at the opposite end of the passageway definedbetween the respective bolts.

As shown in FIGS. 10-12, a generally rectangular air supply bar 230 isprovided for delivering air under low pressure to each of the actuatorsthrough the air inlet opening 200 formed in the support bar 152. The airsupply bar 230 extends along the length of the die with openings alignedwith each of the actuators and has open ends. Air may be supplied ateach open end of the bar 230 from a blower, so that the air flows fromthe ends to the middle, for delivery through each of the actuators.

A threaded nipple 240 has one end threaded into the bar 152 at eachopening 200 and extends outwardly therefrom, as shown in FIG. 11. Thenipple 240 extends into an opening formed in the inner side wall of thebar 230, and is sealed at the bar 152 by a gasket 242. At a plurality ofpositions along the length of the die, the bar is further provided withouter removable access plugs 250 closing an access opening. A flat nut252 is threaded on the exposed end of the threaded nipple 240 in the bar150 opposite each such access opening to retain the air bar 230 in placeon the support bar 152. The openings are then closed by replacing theplug 250.

In the set up of the actuators, the die lip is adjusted to a desiredintermediate position or gap with respect to the opposed die lip, andthe thimble 190 is adjusted and locked into position with respect to theinner and outer die bolts, thereby coupling the outer ends of these diebolts together so that they must move in unison. This is done undersubstantially neutral conditions without applying any substantialtension or compression in the inner die bolt 180 and at some midpointpower level. This allows inward and outward movement of the inner bolt180, to both open and close the die gap.

Thereafter, in operation, the die and its internal passageways areconventionally heated, such as by hot oil and/or electric heating, tomaintain the desired internal die temperature. One or more heating rodsmay also be placed external to the die, along the die lips, asnecessary.

Fine control over the gauge thickness is accomplished by applying acooling flow to the inner bolt 180, through the air bar 230, such as byapplying air at each end from a small air blower, not shown, at aconstant relatively low pressure, such as about 1 cfm/bolt.Simultaneously, heat is applied to the heaters 175 as necessary, and theapplication of heat will cause the moveable die lip to be moved in adirection, by applying tension to the inner bolt 180, so as to widen theextrusion gap as previously described in connection with the embodimentof FIG. 2. The removal of heat at any location, accompanied by theconstant cooling bias on the inner rod, results in the narrowing of thegap at such locations by placing the inner bolt in compression, and thedie lip 110 will be caused to be bent about its axis parallel to itsmounting surface to accommodate such minor adjustments. In this manner,a uniform gauge thickness may be maintained across the full length ofthe die lip.

Since the inner bolt is constantly thermally biased by cooling, theheating of the outer bolt has little effect upon the viscosity of theextruded material since little of the heat from the outer bolt willmigrate into the die lip through the outer bolt and the actuatormounting bar connection. This can be seen by reference to the diagram ofFIG. 13. A die constructed according to FIGS. 7-12 was equipped withthermocouples for measuring temperature at the outer die bolt 170, theinner rod 180, and at the moveable die lip 110. The temperatures werestabilized and then the temperature of the outer die bolt as representedby line 300 on the graph was varied from a maximum of just less than 450to a minimum of about 325 and the temperatures of the inner die bolt asrepresented by graph line 310 and the die lip as measured by graph line320 were recorded while the inner die rod was being cooled with aconstant flow of cooling air, as previously described. It may be seenfrom FIG. 13 that no noticeable excursions of the die lip temperature orthe inner bolt temperature were recorded between the significant maximumand minimum excursions of the outer bolt temperature 300 indicating thatthe temperature of the die lip is maintained relatively independent ofthe actuating temperature of the outer bolt 170.

The method practiced by the invention for controlling the extrusion gapof a slot-type film extruding die, in which at least one longitudinallyextending external die lip is mounted for adjustment on an externalplanar surface of a die body, and in which the die lip may be bent inthe plane for adjusting variations in film gauge, may be described bythe steps of applying a plurality of actuators along the longitudinallength of the external die lip in relatively uniformly spaced relationto each other. The output forces of the actuators are in a common planewhich common plane is parallel to the surface plane of the die body onwhich the die lip is mounted. The relative heating and cooling of eachof the actuators is controlled in accordance with the film gauge desiredat the position of the die lip as represented by the actuator to causeadjusting movements of the die lip accompanied by bending of the dielip. The film gauge is increased by applying heat to the actuators, andis decreased by removing such heat.

While the method herein described, and the forms of apparatus forcarrying this method into effect, constitute preferred embodiments ofthis invention, it is to be understood that the invention is not limitedto this precise method and forms of apparatus, and that changes may bemade in either without departing from the scope of the invention, whichis defined in the appended claims.

I claim:
 1. A thermal actuator system for controlling the position of amoveable die lip associated with the body of a plastic film extruder diewith respect to another die lip to control the width of an extrusion gapbetween the die lips, comprising:an outer annular bolt having an innerend and an outer end, annular bolt inner end body mounted fixed relationto the extruder body, an inner bolt received within said annular boltand having an inner end extending through said annular bolt inner endand having an outer end connected with said annular bolt outer end, thesaid inner end of said inner bolt being attached to said moveable dielip, said inner bolt defining with said annular bolt a coolingpassageway between the outer surface of the inner bolt and the innersurface of said outer bolt, a cooling air inlet at said one end of saidannular bolt opening into said cooling passageway and a cooling airoutlet at the other end of said annular bolt opening into saidpassageway providing for flow of cooling air through said passagewayalong the outside surface of said inner bolt, and a heater associatedwith said annular bolt for heating said annular bolt with respect tosaid inner bolt.
 2. The system of claim 1 in which said moveable die lipcomprises a bar external to and mounted on said die body for slidingmovement on said body slot under the influence of said inner die boltfor varying the width of plastic extrusion through said extrusion gap.3. A thermal actuator system for controlling the position of a moveabledie lip associated with the body of a flat film plastic extruder inwhich the moveable die lip is mounted on an exterior surface of the diebody and is moveable by bending said die lip in a plane parallel to saidsurface in intersecting relation to an extrusion plane through anelongated extrusion slot to control the width of extrusion from suchslot, comprising a plurality of substantially identical heated thermalactuators arranged longitudinally of said die body and each having aforce output end connected to said die lip to cause said die lip to bendin said plane and move along said surface in gauge controlling relationto said extrusion slot, each of said actuators having an outer annularbolt and an inner bolt, said outer annular bolt having an inner endattached to said extruder body and an outer end extending from saidbody, said inner bolt being substantially received within said annularbolt and having an outer end thereof joined with said outer annular boltouter end, said inner bolt inner end extending through the inner end ofsaid annular bolt and joined in force applying relation to said die lipfor directing force substantially parallel to the plane of movement ofsaid die body, said inner and outer bolts defining therebetween acooling passageway extending the length of said outer annular bolt andalong said inner bolt, a cooling inlet at the one end of said annularbolt and opening into said cooling passageway, a cooling outlet at theother end of said outer annular bolt opening into said passagewayproviding for flow of cooling fluid through said passageway, means forapplying cooling fluid under pressure to said inlet for causing saidcooling fluid to move through said passageway along the outer surface ofsaid inner bolt to said outlet, a heater associated with said annularbolt for differentially heating said outer annular bolt with respect tosaid inner bolt so that relative heating of said outer annular boltapplies a tensile force to said inner bolt tending to bend said die lipin said plane to widen said extrusion slot, and relative cooling of saidouter bolt to said inner bolt causes a compressive force to be appliedto said inner bolt tending to bend said die lip in said plane in adirection to close said extrusion slot, said actuator being capable ofwithstanding full expansion and contractive movement of said outerannular bolt without damaging said inner bolt.
 4. In a thermal actuatorfor controlling the position of a moveable die lip forming part of anextruder body for the extrusion of plastic film with respect to theposition of a second die lip, to control the gauge of the plastic film,the improvement comprising an outer annular thermal bolt having an innerend and an outer end, said inner end of said outer annular thermal boltbeing secured to said extruder body, an inner bolt received at leastpartially within said outer annular thermal bolt and having an inner endextending through said annular thermal bolt inner end and connected tosaid moveable die lip and having an outer end, an adjustable connectoron said annular thermal bolt outer end connected to said outer end ofsaid inner bolt, a cooling passageway formed between said outer annularthermal bolt and said inner bolt and extending along the axial length ofsaid outer annular thermal bolt, a cooling fluid inlet formed on one endof said outer annular thermal bolt opening into said cooling passagewayand a cooling fluid outlet formed at the other end of said outer annularthermal bolt providing for flow of cooling fluid through said coolingpassageway along said inner bolt, means for applying a flow of coolingfluid to said cooling fluid inlet for flow through said passageway andsaid cooling fluid outlet, a heater mounted on said outer annularthermal bolt for heating said outer annular bolt with respect to saidinner bolt for causing differential expansions of said bolts accompaniedby movement of said inner end of said inner bolt and said moveable dielip with respect to said extruder body.
 5. The actuator of claim 4further comprising the portion of said inner bolt received within saidouter annular thermal bolt having flat sides extending to corners inwhich said corners form a close fit with the inner surface of said outerannular thermal bolt providing support for said inner bolt againstbuckling.
 6. The method of controlling the extrusion gap of a slot typefilm extruding die in which at least one longitudinally extendingexternal die lip is mounted to a planar exterior surface of the body ofthe die defining a surface plane and is moveable along such surfaceplane for adjusting the extrusion gap, and which die lip may be bentparallel to said plane for adjusting variations in the gauge of the filmextruded from said die comprising the steps of using differentialexpansion bolt actuators in which an outer annular bolt is heated and aninner bolt is cooled by air flow along the length of the space betweensaid bolts thereby providing a variable output force between the outerannular bolt and the inner bolt, applying a plurality of such actuatorsalong the longitudinal length of said external die lip in mutually andrelatively uniformly spaced relation to each other, with the outputforce of each of such actuators directed in a common plane which commonplane is parallel to the surface plane, applying a relatively constantflow of cooling air along the length of said inner bolt, and controllingthe relative heating and cooling of said actuators in accordance withthe film guage desired at the position of said die lip represented bysuch actuator, to cause adjusting movement of said die lip as caused bysaid bending of said die lip.
 7. The method of claim 6 in which the filmgauge is increased by applying heat to said actuators.